ABSTRACT HIV-1 infection begins with the fusion of viral and target cell membranes, which is mediated by the viral envelope glycoprotein upon engagement with host cellular receptors. The HIV-1 envelope glycoprotein undergoes large structural rearrangements during viral entry. There are at least three distinct conformational states of the HIV-1 envelope protein: the prefusion, prehairpin intermediate, and postfusion conformations. Each conformation presents distinct antigenic surfaces to the immune system. In this proposal, we seek to assess the impact of HIV-1 envelope conformation on immunogenicity. We hypothesize that different functional conformers of the HIV-1 envelope protein elicit distinct profiles of antibody responses. A deeper understanding of the relationship between HIV-1 envelope structure and immunogenicity will likely facilitate future structure-based immunogen design strategies at a level of sophistication and structural detail that has not previously been possible. To explore this hypothesis, we propose the following three Specific Aims: 1. We will assess the immunogenicity of the three principal conformational states of gp41 corresponding to the prefusion, prehairpin intermediate, and postfusion conformations in guinea pigs; 2. We will investigate the immunogenicity of uncleaved, precursor state (gp140)3 trimers and cleaved (gp120/gp41ecto)3 trimer complexes in guinea pigs; and 3. We will determine the immunogenicity and protective efficacy of the optimal HIV-1 envelope immunogens in rhesus monkeys.